======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 27, No. 4 Jul 20, 2007 Recently submitted papers: -------------------------- 1) The Origin of the Arches Stellar Cluster Mass Function (Dib et al., MNRAS) 2) Evidence for a Long-Standing Top-Heavy IMF in the Central Parsec of the Galaxy (Maness et al., ApJ) 3) On the origin of kinematic distribution of the sub-parsec young stars in the Galactic center (Yu et al., ApJ) 4) Hypervelocity binary stars: smoking gun of massive binary black holes (Lu et al., ApJL) 5) A Complete Survey of the Central Molecular Zone in NH_3 (Nagayama et al., PASJ) 6) Chemical Abundances of Luminous Cool Stars in the Galactic Center from High-Resolution Infrared Spectroscopy (Cunha et al., ApJ) 7) Cosmic-Ray Heating of Molecular Gas in the Nuclear Disk: Low Star Formation Efficiency (Yusef-Zadeh & Roy, ApJL) ------------------------------------------------------------------------ Email : dib@kasi.re.kr Title : The Origin of the Arches Stellar Cluster Mass Function Author(s): Sami Dib, Jongsoo Kim, and Mohsen Shadmehri Paper : MNRAS, accepted EPrint : ArXiv:0706.0950 Abstract: We investigate the time evolution of the mass distribution of pre- stellar cores (PSCs) and their transition to the initial stellar mass function (IMF) in the central parts of a molecular cloud (MC) under the assumption that the coalescence of cores is important. Our aim is to explain the observed shallow IMF in dense stellar clusters such as the Arches cluster. The initial distributions of PSCs at various distances from the MC center are those of gravitationally unstable cores resulting from the gravo-turbulent fragmentation of the MC. As time evolves, there is a competition between the PSCs rates of coalescence and collapse. Whenever the local rate of collapse is larger than the rate of coalescence in a given mass bin, cores are collapsed into stars. With appropriate parameters, we find that the coalescence-collapse model reproduces very well all the observed characteristics of the Arches stellar cluster IMF; Namely, the slopes at high and low mass ends and the peculiar bump observed at 5-6 M_o. Our results suggest that today's IMF of the Arches cluster is very similar to the primordial one and is prior to the dynamical effects of mass segregation becoming important. ------------------------------------------------------------------------ Email : hmaness@astro.berkeley.edu Title : Evidence for a Long-Standing Top-Heavy IMF in the Central Parsec of the Galaxy Author(s): H. Maness(1), F. Martins(2), S. Trippe(2), R. Genzel(2, 3), J. R. Graham(1), C. Sheehy(4), M. Salaris(5), S. Gillessen(2), T. Alexander(6), T. Paumard(2), T. Ott(2), R. Abuter(2), F. Eisenhauer(2) Institute: (1) Department of Astronomy, University of California at Berkeley, Berkeley, USA (2) Max-Planck Institut f\ddotur extraterrestrische Physik (MPE), Garching, Germany (3) Department of Physics, University of California at Berkeley, Berkeley, USA (4) Department of Astronomy, University of Chicago, Chicago, USA (5) Astrophysics Research Institute, Liverpool John Moores University, Liverpool, UK (6) Faculty of Physics, Weizmann Institute of Science, Rehovot, Israel Paper : ApJ, in press EPrint : 0707.2382 Abstract: We classify 329 late-type giants within 1 parsec of Sgr A^*, using the adaptive optics integral field spectrometer SINFONI on the VLT. These observations represent the deepest spectroscopic data set so far obtained for the Galactic Center, reaching a 50% completeness threshold at the approximate magnitude of the helium-burning red clump (K_S 15.5 mag.). Combining our spectroscopic results with NaCo H and K_S photometry, we construct an observed Hertzsprung-Russell diagram, which we quantitatively compare to theoretical distributions of various star formation histories of the inner Galaxy, using a \chi^2 analysis. Our best-fit model corresponds to continuous star formation over the last 12 Gyr with a top-heavy initial mass function (IMF). The similarity of this IMF to the IMF observed for the most recent epoch of star formation is intriguing and perhaps suggests a connection between recent star formation and the stars formed throughout the history of the Galactic Center. ------------------------------------------------------------------------ Email : lyj@ucolick.org Title : On the origin of kinematic distribution of the sub-parsec young stars in the Galactic center Author(s): Qingjuan Yu^1, mark[3], Youjun Lu^1, \& D. N. C. Lin^1,2 Institute: (1) ^1Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA, ^2Kavli Institute of Astronomy \& Astrophysics, Peking University, Beijing, China Paper : ApJ, in press Abstract: Observations indicate the presence of a massive black hole in the Galactic center. Within a half-parsec from the Galactic center, there is a population of coeval young stars which appear to reside in a coherent disk. Surrounding this dynamically-cool stellar system, there is a population of stars with a similar age and much larger eccentricities and inclinations relative to the disk. We propose a hypothesis for the origin of this dynamical dichotomy. Without specifying any specific mechanism, we consider the possibility that both stellar populations were formed within a disk some 6+/-2 Myr ago. But this orderly structure was dynamically perturbed outside-in by an intruding object with a mass 10^4 M_o, which may be an intermediate-mass black hole or a dark stellar cluster hosting an intermediate-mass black hole. We suggest that the perturber migrated inward to 0.15-0.3 pc from the Galactic center as a consequence of orbital decay under the action of dynamical friction. Along the way, it captured many stars in the outer disk region into its mean-motion resonance, forced them to migrate with it, closely encountered with them, and induced the growth of their eccentricity and inclination. But stars in the inner regions of the disk retain their initial coplanar structure. Quantitatively, a perturber on a low-inclination or overhead orbit to the disk plane can reproduce the observed kinematic structure of these young stars. But this process is unlikely to produce the controversial two-disk structure. We predict that some of the inclined and eccentric stars surrounding the disk may have similar Galactocentric semimajor axis. Future precision determination of their kinematic distribution of these stars will not only provide a test for this hypothesis but also evidences for the presence of an intermediate-mass black hole or a dark cluster at the immediate proximity of the massive black hole at the Galactic center. ------------------------------------------------------------------------ Email : lyj@ucolick.org Title : Hypervelocity binary stars: smoking gun of massive binary black holes Author(s): Youjun Lu(1), Qingjuan Yu(1), \& D. N. C. Lin(1,2) Institute: (1) Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA (2) Kavli Institute of Astronomy and Astrophysics, Peking University, Beijing, China Paper : ApJL, in press Abstract: The hypervelocity stars recently found in the Galactic halo are expelled from the Galactic center through interactions between binary stars and the central massive black hole or between single stars and a hypothetical massive binary black hole. In this paper, we demonstrate that binary stars can be ejected out of the Galactic center with velocities up to 10^3 km/sec , while preserving their integrity, through interactions with a massive binary black hole. Binary stars are unlikely to attain such high velocities via scattering by a single massive black hole or through any other mechanisms. Based on the above theoretical prediction, we propose a search for binary systems among the hypervelocity stars. Discovery of hypervelocity binary stars, even one, is a definitive evidence of the existence of a massive binary black hole in the Galactic center. ------------------------------------------------------------------------ Email : nagayama@astro.sci.kagoshima-u.ac.jp Title : A Complete Survey of the Central Molecular Zone in NH_3 Author(s): Takumi Nagayama(1), Toshihiro Omodaka(2), Toshihiro Handa(3), Hayati Bebe Hajra Iahak(1), Tsuyoshi Sawada(4), Takeshi Miyaji(5), and Yasuhiro Koyama(6) Institute: (1) Graduate School of Science and Engineering, Kagoshima University,, 1-21-30 K\hatorimoto, Kagoshima 890-0065 (2) Faculty of Science, Kagoshima University, 1-21-30 K\hatorimoto, Kagoshima 890-0065 (3) Institute of Astronomy, University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015 (4) Nobeyama Radio Observatory, National Astronomical Observatory of Japan,, Minamimaki, Minamisaku, Nagano 384-1305 (5) Mizusawa VERA Observatory, National Astronomical Observatory of Japan,, 2-21-1 Osawa, Mitaka, Tokyo 181-8588 (6) Kashima Space Research Center, National Institute of Information and Communications Technology,, 893-1 Hirai, Kashima, Ibaraki 314-8510 Paper : PASJ, OCT 2007, Vol.59, No.5 EPrint : 0707.0911 Web : http://jp.arxiv.org/abs/0707.0911 Abstract: We present a map of the major part of the central molecular zone (CMZ) of simultaneous observations in the NH_3 (J,K) = (1,1) and (2,2) lines using the Kagoshima 6-m telescope. The mapped area is \timeform-1D.000 <= l <= \timeform1D.625, \timeform-0D.375 <= b <= \timeform+0D.250. The kinetic temperatures derived from the (2,2) to (1,1) intensity ratios are 20-80 K or exceed 80 K. The gases corresponding to temperature of 20-80 K and >= 80 K contain 75% and 25% of the total NH_3 flux, respectively. These temperatures indicate that the dense molecular gas in the CMZ is dominated by gas that is warmer than the majority of the dust present there. A comparison with the CO survey by \citetsawada shows that the NH_3 emitting region is surrounded by a high pressure region on the l-v plane. Although NH_3 emission traces dense gas, it is not extended into a high pressure region. Therefore, the high pressure region is less dense and has to be hotter. This indicates that the molecular cloud complex in the Galactic center region has a ``core'' of dense and warm clouds which are traced by the NH_3 emission, and an ``envelope'' of less dense and hotter gas clouds. Besides heating by ambipolar diffusion, the hot plasma gas emitting the X-ray emission may heat the hot ``envelope''. ------------------------------------------------------------------------ Email : sellgren@astronomy.ohio-state.edu Title : Chemical Abundances of Luminous Cool Stars in the Galactic Center from High-Resolution Infrared Spectroscopy Author(s): Katia Cunha Kris Sellgren Verne V. Smith Solange V. Ramirez Robert D. Blum Donald M. Terndrup Institute: (1) National Optical Astronomy Observatory and Observatorio Nacional, Rio de Janeiro, Brazil (2) Department of Astronomy, The Ohio State University (3) National Optical Astronomy Observatory (4) Infrared Processing and Analysis Center, California Institute of Technology (5) National Optical Astronomy Observatory (6) Department of Astronomy, The Ohio State University Paper : ApJ, in press EPrint : 0707.2610 Web : http://xxx.lanl.gov/abs/0707.2610 Abstract: We present chemical abundances in a sample of luminous cool stars located within 30 pc of the Galactic Center. Abundances of carbon, nitrogen, oxygen, calcium, and iron were derived from high-resolution infrared spectra in the H- and K-bands. The abundance results indicate that both [O/Fe] and [Ca/Fe] are enhanced respectively by averages of +0.2 and +0.3 dex, relative to either the Sun or the Milky Way disk at near solar Fe abundances. The Galactic Center stars show a nearly uniform and nearly solar iron abundance. The mean value of A(Fe) = 7.59 +/- 0.06 agrees well with previous work. The total range in Fe abundance among Galactic Center stars, 0.16 dex, is significantly narrower than the iron abundance distributions found in the literature for the older bulge population. Our snapshot of the current-day Fe abundance within 30 pc of the Galactic Center samples stars with an age less than 1 Gyr; a larger sample in time (or space) may find a wider spread in abundances. ------------------------------------------------------------------------ Email : zadeh@northwestern.edu Title : Cosmic-Ray Heating of Molecular Gas in the Nuclear Disk: Low Star Formation Efficiency Author(s): F. Yusef-Zadeh, M. Wardle and S. Roy Paper : ApJL, in press EPrint : 0707.0910 Abstract: Understanding the processes occurring in the nuclear disk of our Galaxy is interesting in its own right, as part of the Milky Way Galaxy, but also because it is the closest galactic nucleus. It has been more than two decades since it was recognized that the general phenomenon of higher gas temperature in the inner few hundred parsecs by comparison with local clouds in the disk of the Galaxy. This is one of the least understood characteristics of giant molecular clouds having a much higher gas temperature than dust temperature in the inner few degrees of the Galactic center. We propose that an enhanced flux of cosmic-ray electrons, as evidenced recently by a number of studies, are responsible for directly heating the gas clouds in the nuclear disk, elevating the temperature of molecular gas ( 75K) above the dust temperature ( 20K). In addition we report the detection of nonthermal radio emission from Sgr B2-F based on low-frequency GMRT and VLA observations. The higher ionization fraction and thermal energy due to the impact of nonthermal electrons in star forming sites have important implications in slowing down star formation in the nuclear disk of our galaxy and nuclei of galaxies. ------------------------------------------------------------------------ (Older versions of the Newsflash can be found at the gcnews web-page) ======================================================================== Edited by Sera Markoff, Loránt Sjouwerman, Joseph Lazio, Cornelia Lang, Rainer Schödel, Masaaki Sakano, Feng Yuan - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please follow the instructions which are at http://www.aoc.nrao.edu/~gcnews/home/submission.shtml ========================================================================